Add custom page up/down keyboard selection in song select

[AndyBodnar]

This implements custom page up/down handling in song select like peppy mentioned in the issue. Instead of using the standard scroll container behavior which just scrolls the view by a page, it now changes the keyboard selection by multiple items at once.

The page navigation moves the selection by 5 visible items at a time (this is configurable via the PageSelectionDistance property if we want to tweak it later). The implementation doesn't wrap around at the ends of the list, so pressing Page Down at the bottom just stays at the bottom rather than jumping back to the start.

I've also added some basic tests to verify the page navigation moves the selection by more than one item and doesn't wrap around.

Closes #36099

[peppy]

Duplicate of #36293. Only one of these two should be merged.

[AndyBodnar]

@peppy Thanks for flagging #36293 - I hadn't seen it when I started working on this.

A few differences in my implementation that might be worth considering:

Tests - This PR includes TestSceneBeatmapCarouselPageNavigation.cs with test coverage for the page navigation behavior

Configurability - Added a PageSelectionDistance property so the "items per page" can be tweaked later without code changes

Lines: +292 (vs +85) - the extra is mostly test coverage

That said, I'm happy to go whichever direction works best for the project. If you'd prefer to move forward with #36293, I can close this - just point me in the direction you want. Alternatively, I could contribute my tests to their PR if that's useful.

[AndyBodnar]

Closing this one since #36293 covers the same feature. Makes more sense to focus efforts on a single PR rather than splitting review attention.

If the other PR needs any help with tests or anything else, happy to contribute there.